AW: multiple power supplies in a modular

[Mark Amundson]

> Mark said:  ...and get 10 to 100uf of low ESR tantalum capacitors
> across the supply rails after the regulators.
> 
> I would not use tants. When these die due to a fault or whatever, they
> tend to go short. If you conect these directly across your rails, then
> its goodbye regulator and hello to burnt pcbs. Ordinary electrolytics
> (elec's...?) are fine for this I would have thought, especially if you
> put a low value cap in parallel with it. Tony Allgood

Ok, I understand your concern. But hands up anybody who has seen a
tantalum, within rated voltage, fry a board. Now I know electrolytics
have some self healing ability to over voltage punch throughs, but a
regulated supply with some capacitance tends to be fairly safe from
voltage spikes. The "well designed" caveot applies.

The rationale for tantalums over low ESR electroytics is that the ESR
knee for tantalums will take me into several megahertz which allows the
ceramic caps to take over the low reactance task. In fact, I like to
switch down to .01uf on digital circuits to keep filtering intact up to
the VHF band. Most good regulators will respond to supply variations up
to 40kHz and then the capacitors must do the filtering. I worry that
switching supply electrolytics "knee out" in reactance before 1 MHz
leaving a chance for high frequencies riding on the regulated lines. A
little over-kill, but I have peace of mind. Besides I'd still have to
deal with electrolytic dry-out and 10,000 hour operational life rating
issues.

Mark Amundson,